The present invention generally refers to prostheses, and more particularly to a hip prosthesis for total replacement of a hip joint, including a shank anchored in the bone canal of the femur and a saddle-shaped head connected to the shank and rotationally movable with respect to the shank with at least two degrees of freedom, with the saddle-like head being formed with a bearing seat for providing a support for a portion of a pelvic bone.
European Pat. No. EP-A-0 300 131 A1 describes a saddle-type prosthesis, with the saddle-shaped head being rotatable relative to the shank about a transverse axis which extends substantially in longitudinal direction of the leg and is positioned parallel offset with respect to the longitudinal shank axis. Thus, the saddle-shaped head is rotationally movable relative to the shank with one degree of freedom in order to enable an outer or inner rotation of the leg. Additional rotational movements and pivot movements of the leg, i.e. bending and stretching or abduction and adduction are effected by this saddle-type prosthesis through a motion between the pelvic bone and saddle-shaped head, whereby the seat of the saddle-shaped head is flat and open to enable movements in longitudinal direction thereof as well as transversely thereto. Thus, in conjunction with the pivot joint that allows outer and inner rotations within the prosthesis, the leg movement attained by this type of prosthesis resembles the mobility achieved by a ball joint, so that such saddle-type prostheses have been frequently used for replacement of hip joints. Practice has shown however that the saddle-shaped head, as a result of the relative open configuration of the seat, has a tendency during the initial months after surgery to migrate from its bony bearing or to become disengaged therefrom. Later on, this phenomena is not experienced as typically a tight tissue capsule will build up around the metallic saddle-shaped head and ossify in many cases. In addition, the relatively small seat or bearing surface of the pelvic bone upon the saddle-shaped head may also cause slight upward migration of the saddle-shaped head in the pelvic bone, or even a fracture of the pelvic bone above the saddle-shaped head when the bone is subject to excessive stress.
European Pat. No: EP-A-0 300 131 A1 further discloses a saddle-type prosthesis of a type having a ball joint additionally disposed beneath the saddle-shaped head at a location between the saddle-shaped head and the shank, and permitting to a limited degree rotational movements about further axes that are inclined relative to the transverse saddle axis so that the saddle-shaped head is rotationally mobile relative to the shank with three degrees of freedom. The superimposed disposition of two joints, i.e. the ball joint, on the one hand, and the joint between the open saddle-shaped head and the pelvic bone, on the other hand, lead however to instability of the saddle-shaped head that may cause a tilting. Therefore, this saddle-type prosthesis has not been used in practice.